Remote method of measuring subsurface water temperatures

ABSTRACT

A technique for measuring the unknown subsurface temperature T s  of a bulk transparent medium such as ocean water by splitting a pulsed laser beam having a high intensity into two sub-beams, one of which is a probe beam directed into the medium. The intensity of the output beam pulses exceeds a predetermined threshold sufficient to cause stimulated Brillouin scattering within the medium and to produce therefrom a phase-conjugate beam which propagates along the path of the first sub-beam but in the opposite direction. The second sub-beam combines with the PC beam and the combined beams mix at the cathode of a photodetector thereby producing a heterodyne frequency proportional to the temperature T s . Converts the heterodyne frequency into a temperature value yields the desired unknown T s .

This invention was made with Government support under Contract No. N00014-87-C-0739 awarded by the Department of the Navy. The Government has certain rights in this invention.

RELATED APPLICATIONS

The invention described herein relates generally to the following patent applications:

"APPARATUS FOR AND METHOD OF REMOTELY SENSING SUB-SURFACE WATER WATER TEMPERATURES," U.S. Ser. No. 064,371, filed June 27, 1987 now U.S. Pat. No. 4,867,564.

"METHOD OF REMOTELY DETECTING SUBMARINES USING A LASER," U.S. Ser. No. 064,375, filed June 22, 1987 now U.S. Pat. No. 4,867,558.

"REMOTE SUBSURFACE WATER TEMPERATURE MEASURING APPARATUS," U.S. Ser. No. 387,735, filed Aug. 1, 1989.

BACKGROUND OF THE INVENTION

This invention relates to the remote measurement Of the properties of transparent media such as subsurface ocean temperatures profiles and in particular to an improved method of remotely measuring such temperature profiles from surface or subsurface vessels or aircraft.

There are several applications for remotely sensing or measuring the temperature of a bulk transparent medium such as water. One of such applications is the sounding of temperature profiles in the ocean which is useful for a variety of oceanographic purposes such as measuring the depth of the thermocline, sensing internal waves, measuring heat content of oceans for meteorological applications and mapping acoustical propagation paths sensitive to temperature gradients. Insitu temperature sensors such as thermistors, thermocouples, etc. have been used in the past for these purposes but, because they are not remote sensors, are slow and awkward. A remote sensing technique in wide use is the monitoring of thermal radiation; this technique, however, is limited to measuring predominately surface temperatures.

This invention is directed to an improved technique for remotely measuring temperature within, i.e., below the surface of suitable transparent media or substances, for example sea water.

OBJECTS AND SUMMARY OF THE INVENTION

A general object of the invention is the provision of a method of measuring the subsurface temperature of a transparent substance remotely and substantially instantly.

Another object is the provision of such a technique that has a high signal-to-noise ratio and therefore produces highly accurate measurements.

A further obJect is the provision of such a method in which the irradiance in the received return signal from the target substance is relatively high.

These and other objects of the invention are achieved by generating a high power pulsed laser beam capable of producing stimulated Brillouin scattering (SBS), splitting this beam into two sub-beams and directing one sub-beam, designated the probe beam, into the transparent medium of unknown temperature and thereby producing a phase-conjugate beam from the medium. The return phase-conjugate beam and the other sub-beam are mixed together to derive a difference frequency proportional to the unknown temperature. The difference frequency value is then converted to a value indicative of the unknown temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic drawing of apparatus used in the practice of the subject invention.

FIGS. 2A through 2F are representations of several actual heterodyne waveforms as a function of water temperature at 5° C. intervals as derived from the practice of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the drawings, FIG. 1 illustrates temperature measuring apparatus 10 in which a pulsed laser 12 generates an intense output beam 13. Beam 13 is incident on and split by a beam splitter 14 into a first sub-beam 16 which passes through splitter 14 and a second sub-beam 17 which is reflected thereby. Sub-beam 16 passes through a lens or telescope 18 to a mirror 19 and is focussed thereby to a focal point 20 in a transparent medium 22 such as water. Telescope 18 is adjustable so as to enable adjustment of the location of focal point 20 to various selected locations or depths within medium 22 as suggested by the proximate double-headed arrow. The unknown temperature of interest, designated T_(s), is the temperature of medium 22 at focal point 20.

Sub-beam 17 propagates from splitter 14 to and is reflected by a mirror 24 along the same path back to splitter 14. The spacing of mirror 24 from splitter 14 is variable as indicated by the adjacent double-headed arrow.

The intensity of the pulsed output beam 13, i.e., the intensity or irradiance of the pulses from laser 12 exceeds a predetermined threshold level at which sub-beam 16 produces stimulated Brillouin scattering in cell 22 resulting in the generation of phase-conjugate or "time-reversed" beam whose ray runs along the same trajectory as but in opposite direction to sub-beam 16. The direction of the PC beam is designated by the arrow 16_(pc). This phenomenon, called optical phase conjugation, is well known and is described in detail in articles entitled Optical Phase Conjugation by V. V. Shkunov et al, Scientific American, pages 54-59 (September 1985) and Applications of Optical Phase Conjugation by D. M. Pepper, Scientific American, pages 74-83 (January 1986).

The Brillouin backscatter optical wave from a spontaneously generated phonon interacts with the probe beam and the interaction produces a traveling electric field that travels exactly at the sound velocity in the particular substance. If the light intensity is great enough and the electrostrictive coefficient of the medium is sufficiently strong, this traveling electric field will create an acoustic field which aligns itself in a column with the probe beam and the backscattered beam at the expense of scattering in other directions. Hence the collection of light is more efficient in the SBS case.

The presence of the induced electric field traveling at the sound velocity can be derived in the following way. Assume that an electromagnetic wave (the probe laser) propagates in a substance in the+× direction with velocity c, then the electric field may be written as

    E.sub.1 =E.sub.o sin(ω.sub.1 t-ω.sub.1 X/C)    (1)

where,

E_(o) =magnitude of the wave,

ω=frequency, and

t=time parameter,

and assume that a wave of different frequency (the Brillouin scattering) propagates in the -x direction, the electric field of which is written as,

    E.sub.2 =E.sub.o sin(ω.sub.2 t+ω.sub.2 X/C),   (2)

where,

ω₂ =frequency of the second wave.

The total electric field in their common region is

    E=E.sub.1 +E.sub.2 =E.sub.o [sin(ω.sub.1 t-ω.sub.1 X/C)+ sin(ω.sub.2 t+ω.sub.2 X/C)]                   (3)

which can be arranged by trigonometric identities to the form

    E=2E.sub.o {sin1/2[(ω.sub.1 +ω.sub.2)t-(ω.sub.1 -ω.sub.2)X/C]}{cos1/2[(ω.sub.1 -ω.sub.2)t-(ω.sub.1 +ω.sub.2)X/C]}                                      (4)

which is of the form of a high frequency (ω₁ +ω₂) signal modulated with a low frequency (ω₁ -ω₂) envelope. The velocity of the envelope is found by making its argument constant, i.e.,

    (ω.sub.1 -ω.sub.2)t-(ω.sub.1 +ω.sub.2)X/C=K. (5)

The envelope velocity is therefore

    dx/dt=(ω.sub.1 -ω.sub.2)C/(ω.sub.1 +ω.sub.2) =(f.sub.1 -f.sub.2)C/(f.sub.1 +f.sub.2).                  (6)

In the case of Brillouin scattering f₂ has been produced by Doppler shift by interacting with an acoustic wave at velocity v. The Doppler equation, namely

    f.sub.2 =f.sub.1 (c-v)/(c+v),                              (7)

which when solved results in the following expression:

    c=(f.sub.1 =f.sub.2)v/(f.sub.1 -f.sub.2).                  (8)

A substitution into the prior expression (6) for dx/dt yields dx/dt=v. The envelope of the composite electric field wave therefore travels exactly at the acoustic velocity.

In order to achieve SBS, a predetermined threshold intensity level for the optical probe beam must be exceeded. This intensity level must be sufficient so that the following relationship exists:

    exp[GIL]≧10.sup.13

or,

    exp[GIL]≧exp[30],

    (since 10.sup.13 =exp[30]).

Or more simply, the intensity, I≧30/GL, where,

G=a gain parameter which is a property of the medium, m/W

I=intensity of the optical probe beam (W/m²) and,

L=interaction length, m,

see Principles of Phase Conjugation by Zel'dovich et al., Springer-Verlag, Vol. 42, page 29, Springer Series on Optical Sciences (Springer Verlag Berlin Heidelberg, 1985). (For water G is typically 5×10⁻¹¹ m/W.)

The sound velocity in water is related to the temperature and salinity and is given in the article entitled Development of Simple Equations for Accurate and More realistic Calculation of the Speed of Sound in Sea Water by C. C. Leroy, Journal of Acoustical Society of America, No. 216, page 216 (1969):

    v=1492.9+3(T-10)-0.006(T-10).sup.2 -0.04(T-18).sup.2 +1.2(S-35)-0.01(T-18)(S-35)+Z/61                          (9)

where,

T=temperature in degrees C,

S=salinity in parts per thousand,

Z=depth in meters.

The phase conjugate of sub-beam 16, designated 16_(pc), propagates from medium 22 precisely along the path of sub-beam 16 except in the opposite direction to mirror 19 and is reflected thereby through lens 18 to splitter 14 from which it is reflected to mirror 26 and ultimately to photodetector 27, such as a photodiode. Sub-beam 17 reflected from mirror 24 passes through splitter 14 to mirror 26 and is reflected thereby to photodetector 27. It is important that the pulses comprising sub-beam 17 and the pulses comprising PC beam 16_(pc) arrive at the active mixing surface of photodetector 27 at the same time for effective heterodyning of these two signals. Since there is no optical storage mechanism, if the pulses from the two beams are not properly timed to arrive at the photodetector at substantially the same instant, the two pulses will not mix with each other. To this end, the optical spacing of both focal point 20 and mirror 24 from photodetector 27 are the same. The adjustable spacing between mirror 24 and splitter 14 accommodates this requirement. The adjustment of the location of focal point 20 in medium 22 may also be directly related to adjustment of the position of mirror 24 by means of a mechanical, electromechanical or other suitable link or connection between lens 18 and mirror 24 as indicated by line 28 between these components.

A phase-conjugated beam derived from SBS has its optical frequency shifted by a frequency that produces an acoustical wavelength in the water equal to half the optical wavelength, i.e. ##EQU1## where, Δν=the optical frequency shift,

^(v) a=the acoustic velocity in water,

n=the index of refraction in water, and

λ=the wavelength of the incident beam in a vacuum.

Photodetector 27 mixes sub-beam 17 and beam 16_(pc) and produces at its output 29 a difference frequency that is proportional to temperature T_(s). This frequency may be as high as 8 GHz. In order to enable conversion of a difference frequency of this magnitude to an analog temperature value with conventional oscilloscope/recorder equipment, it is desirable to further mix photodetector output 29 to down convert the latter to a lower more manageable frequency. This is accomplished with down converter equipment enclosed within broken lines 31 and comprising a filter 32, an amplifier 33, a signal generator 34, a mixer 35, an amplifier 36 and a filter 37. High-pass filter 32 rejects the basic pulse components in output 29 but preserves the SHF (super high frequency) signal. Amplifier 33 increases the signal level into double-balanced mixer 35 driven by signal generator 34. In fresh water the SHF signal is typically 7.1 to 7.5 GHz; setting the signal generator to 6,550 MHz produces an IF (intermediate frequency) between 550 and 950 MHz for convenient viewing on a real time oscilloscope (39) such as Tektronix Model No. 7104. The output 38 of filter 37 is observed by scope 39, and is converted to a voltage by a wide band delay line discriminator 40 which feeds a chart recorder 41. Delay line discriminator 40 is designed to operate between 500 to 1,000 MHz.

FIG. 2 shows a series of photographs of the display from oscilloscope 39 depicting the heterodyne waveform as a function of medium (water) temperature from 5° to 30° C. at 5° intervals. The change in frequency is clearly seen in these photographs at a high signal-to-noise ratio.

While the invention has been described with reference to its preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for the elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teaching of the invention without departing from its essential teaching. 

What is claimed is:
 1. An optical method for remotely measuring an unknown temperature T_(s) of a transparent medium, consisting of the steps of:directing into said medium a pulsed laser beam comprising a plurality of pulses each having an intensity sufficient to cause stimulated Brillouin scattering in said medium and thereby causing a phase-conjugate beam to emanate from said medium; adjustably focussing said pulsed laser beam to a plurality of focal points at different depths within said medium; adjusting the range of a portion of said pulsed laser beam and said phase-conjugate beam to enable mixing of the pulses of each beam at the same time; mixing said phase-conjugate beam with said portion of said pulsed laser beam and thereby producing a difference frequency proportional to temperature T_(s;) and converting said difference frequency to a value of temperature corresponding to T_(s).
 2. The method of remotely measuring an unknown subsurface temperature T_(s) of a body of water, consisting of the steps of:generating a pulsed laser output beam having an intensity exceeding the intensity required to produce stimulated Brillouin scattering in said body; splitting said output beam into first and second sub-beams; directing said first sub-beam into said body and producing therefrom a phase-conjugate beam; adjustably focussing said first sub-beam to a plurality of focal points at different depths within said body; adjusting the range of said second sub-beam from said photodetector to be equal to the respective ranges of said focal points from said photodetector, mixing said PC beam and said second sub-beam in a photodetector and producing a difference frequency proportional to T_(s;) and converting said difference frequency into a value of T_(s).
 3. The method according to claim 2 with the steps of:repeating said generating step while moving said beam across said body of water; and plotting, after said converting step, the values of T_(s) verses distance thereby generating a temperature profile plot.
 4. The method according to claim 3 wherein said pulsed output beam has a frequency in the blue-green spectrum. 